The last several years have witnessed an explosion in our understanding of vertebrate development, largely based on work from a few model genetic organisms. The embryonic zebrafish is one of these, and many research grants are funded by NIH and NIEHS to utilize this model. Studies of the embryology and genetics of zebrafish and the recent identification of 1,000's of genetic mutations have lead to this dramatic increase in the number of laboratories (now ca 450) using zebrafish to study the basic mechanisms of vertebrate developmental biology. Although zebrafish have become an extremely important research model, relatively little is known about the diseases that affect them when held in captivity. Indeed, severe health problems in zebrafish research colonies could seriously jeopardize many millions of dollars in research funding, and some research facilities have experienced devastating, acute mortalities in their zebrafish colonies. Moreover, persistent, but less severe, infections have plagued several facilities. As with other laboratory animals used in research, it is imperative to conduct studies with disease-free, healthy zebrafish. This is even more important now as several researchers are now using zebrafish as models in long-term studies on human pathogens and in toxicology. Unlike rodent models, there are no certified specific pathogen free (SPF) stocks of zebrafish, and we are just now developing sensitive diagnostic tests for the most serious pathogens. The major aim of the present study is to improve the health of zebrafish used in research facilities by elucidation of the pathogenesis and modes of transmission of the most important pathogens in this fish - e.g., Pseudoloma neurophilia, a microsporidium of the central nervous system that is very prevalent in zebrafish, and Mycobacterium spp, the cause offish tuberculosis. P. neurophilia is extremely prevalent in zebrafish facilities and evidence indicates that it is transmitted vertically. We will therefore development of a non-lethal diagnostic test for brood fish, and investigate methods to disinfect eggs. For mycobacteria, we will determine which strains or species are most virulent, as we see dramatic differences in mortalities between outbreaks. PCR tests will be made for the virulent strains. Results will be quickly distributed to the zebrafish community through ZFIN, peer reviewed publications, and our web based manual on zebrafish diseases